This application claims priority to Japanese Application No. 2008-126621 filed May 14, 2008, the entire contents of which are hereby incorporated by reference.
1. Technical Field
The present invention relates to magnetic sensors, and in particular, to magnetic sensors using magneto resistive effect elements.
2. Related Art
Magnetic sensors capable of detecting changes in magnetic fields have been developed as measurement devices and used in various applications including ammeters and magnetic encoders. The NVE CORPORATION GMR Sensor Catalog (Non-Patent Document 1) discloses examples of such magnetic sensors, in which GMR (Giant Magneto Resistive effect) elements are used as elements for detecting changes in magnetic fields. A GMR element is an element in which a resistance value to be output varies depending on input magnetism. According to the output resistance value, a detected change in a magnetic field can be measured.
As shown in Non-Patent Document 1, a specific exemplary configuration of a magnetic sensor using GMR elements includes four GMR elements arranged on a substrate and constituting a bridge circuit. By detecting a differential voltage in the circuit, changes in resistance values of the GMR elements due to changes in the object magnetic field are detected. With this configuration, a sensor having high sensitivity in changes in a magnetic field can be realized.
More specifically, the magnetic sensor disclosed in Non-Patent Document 1 includes a GMR chip (magnetic field detection chip) using spin-valve type GMR elements as elements for detecting changes in a magnetic field, in which resistance values to be output vary corresponding to the direction of the magnetic field to be input. Each GMR element is fixedly magnetized in a predetermined direction on one surface thereof so as to be able to detect a magnetic field in a predetermined direction. In order to reduce the size of the GMR chip and variations in the respective resistance values of the GMR elements, four GMR elements constituting a bridge circuit are formed a single GMR chip. As such, the fixed magnetization directions of all four GMR elements are the same.
In the case of detecting a magnetic field in one direction using a GMR chip having such a bridge circuit, as changes in the magnetic field are almost the same in all GMR elements, changes in the resistance values output from the respective GMR elements according to the changes in the magnetic field are almost the same. As such, it is difficult to detect a differential voltage in the bridge circuit. In order to solve this problem, in Non-Patent Document 1, two of the four GMR elements are covered with a NiFe film. As the GMR elements covered with the NiFe film cannot detect changes in the magnetic field, the resistance values of the other two GMR elements vary in accordance with the changes in the magnetic field. Thereby, a differential voltage can be detected in the bridge circuit, so that changes in the magnetic field can be detected.
[Non-Patent Document 1] NVE CORPORATION (US), “NVE CORPORATION GMR Sensor Catalog”, [online], P. 7, [Searched on Mar. 17, 2008], the Internet <URL:http://www.nve.com/Downloads/catalog.pdf>
However, the art disclosed in Non-Patent Document 1 involves a problem that as the resistance values of the two GMR elements, among the GMR elements constituting the bridge circuit, covered with the NiFe film do not vary, a large differential voltage is not output, so that the magnetic field cannot be detected with high accuracy.
On the other hand, in the case of using a bridge circuit including GMR elements as described above in order to detect a magnetic field in one direction with high accuracy, it is necessary to separately arrange the GMR elements in two and two, for example, so that the GMR elements have different fixed magnetization directions. However, in the configuration of the GMR elements being separated, a bridge circuit is constituted of a plurality of chips, causing errors in the resistance values of respective GMR elements. As such, as an offset voltage is caused even when the magnetic field is zero, measurement cannot be performed with high accuracy. Further, to form a bridge circuit, connecting points and wires are required for connecting GMR elements formed on a plurality of chips. This increases the resistance, whereby the offset voltage may further be increased. Furthermore, if the bridge circuit is constituted of a plurality of chips, it is necessary to include connecting points, wires, and substrates as described above, causing a problem that the size of the sensor cannot be reduced.
Further, if the offset voltage is large when the magnetic field is zero, the temperature characteristics of the offset voltage deteriorate. This means, although the resistance values of the four GMR elements increase or decrease in accordance with rise or drop of the temperature, variations therein become larger because the GMR elements are separately provided as described above. Consequently, a large offset drift is caused, so that it is more difficult to detect the magnetic field with high accuracy.
Further, in the case where the GMR elements are separately provided, they cannot be arranged densely. With this configuration, as variations are caused in the film thickness of the GMR elements, variations in the resistance of the respective GMR elements become large, whereby the offset voltage becomes larger, as the case described above. Further, when the GMR elements are separately provided, a pad must be formed for each element and the element area becomes larger, so that it is difficult to reduce the size of the chip. This causes a problem that the number of chips which can be manufactured in the manufacturing steps decreases, so that the product price increases.